1. Field of the Invention
The present invention in general relates to a transmission mechanism, in particular, to a transmission mechanism for a linear actuator.
2. Description of Prior Art
Linear actuators mainly use a plurality of driving assemblies, for example, motor, worm and worm wheel, to rotate a guide screw. In the meantime, a telescopic tube is engaged with the guide screw and is driven thereby to perform the linear motions of stretch and retraction. Since there are many superior conditions of the linear actuator, for example, simplicity and convenience, compared to those of air/oil pressure cylinders in structure and arrangement, the linear actuator has been comprehensively applied to the facilities of hospital bed and electric chair or other occasions needed the operations of stretch and retraction.
According to prior arts, in order to increase mechanism efficiency or possess faster moving speed, it usually has to enlarge the guide route of the guide screw. However, there are two problems arisen subsequently. First of all, the guide screw is unable to be self-locked. Secondly, the declining speed of the nut engaged with the guide screw and the telescopic tube connected to the nut is too fast, causing an uncomfortable problem to the user.
According to a prior art disclosed in European patent publication-EP 1556630B1, the actuator includes a reversible motor, a worm and worm wheel set, an adjustable assembly, a cylindrical portion and a coil spring. The worm and worm wheel set is respectively connected to the motor and the adjustable assembly for operation. The coil spring is arranged to the cylindrical portion externally and has a specific winding direction, such that the coil spring has a braking effect imposing on the adjustable assembly with respect to a specific motion direction, whereby the coil spring is to tighten on the surroundings of the cylindrical portion. By so doing, the coil spring is positioned on the static worm and worm wheel set with one side fixed at the worm wheel, making the coil spring rotate around the static cylindrical portion fixed.
However, according to this kind of actuator, during braking process, a relative motion is arisen between the coil spring and the cylindrical portion to generate frictional heat, which has high temperature and is easy to cause melting on the cylindrical portion. Furthermore, when the guide route of the guide screw becomes larger, the braking power needed by the coil spring is greater. In this case, a high temperature melting is easier to occur. In addition, when there is no loading, the braking force is still existed. This means that the motor still has to overcome the resistance of the coil spring when a retraction is performed. So, it is still needed to consume power even when there is no loading, causing an unnecessary waste of power, which is a drawback needed to be solved.
Accordingly, after a substantially devoted study, in cooperation with the application of relative academic principles, the inventor has finally proposed the present invention designed reasonably to possess the capability to improve the drawbacks of the prior art significantly.